1. Field of the Invention:
This invention relates to a high optical quality transparent sheet of plastic containing at least one base layer of a polyurethane with energy-absorbing properties, which can be used alone or in combination with other materials, in particular, in laminated glazings, where it is combined with a monolithic or laminated glass and/or plastic support such as a vehicle windshield.
2. Description of the Background:
A high optical quality transparent sheet is known from European Patent Publication No. 0 133 090 which sheet can be used alone or in association with other materials particularly in the manufacture of laminated glazings. This sheet contains a layer formed in a continuous process by reactive pouring onto a flat horizontal support, from which it is detachable, of a reaction mixture of polyurethane components. The term "reactive pouring" means pouring in the form of a layer or a film of a liquid mixture of the components in the monomeric or prepolymeric state, following by the polymerization of said mixture using heat.
The reaction mixture described in the above publication contains a component with active hydrogens, in particular a polyol component, and an isocyanate component containing at least one aliphatic or cycloaliphatic diisocyanate or a diisocyanate prepolymer, with this component having a viscosity of below approximately 5000 centipoises at +40.degree. C. The polyol component contains at least one difunctional long polyol with a molecular weight of between 500 and 4000, at least one short diol as a chain extending agent and, as necessary, a small amount of at least one polyol with a functionality of greater than two, in particular aliphatic triols.
The proportions of the long polyol, the short diol and optionally the polyol with a functionality of greater than 2 are generally selected such that for a hydroxyl equivalent, the long polyol represents approximately from 0.30 to 0.45 equivalent, the short diol approximately 0.2 to 0.7 equivalent and the polyol with a functionality of greater than 2 approximately 0 to 0.35 equivalent. Under these circumstances, the layer has the following mechanical characteristics, measured in accordance with AFNOR/NFT norms 46 002, 51 034 and 54 108.
a flow threshold stress .sigma..sub.y at -20.degree. C. less than or equal to 3 daN/mm.sub.2, PA0 a breakage stress .sigma.R at +40.degree. C. greater than or equal to 2 daN/mm.sub.2, PA0 a stretching to breakage .epsilon..sub.R at +20.degree. C. of between 250 and 500%, PA0 a resistance to primed tearing Ra at +20.degree. C. greater than or equal to 9 daN/mm of thickness.
In addition to energy-absorbing properties, as indicated above, this layer has scratch- and abrasion-resistant properties which render it capable of being used as an external layer under certain conditions. Thus, it has a scratch-resistance of greater than 20 grams and an abrasion-resistance with a blur separation of less than 4%, said values being measured in accordance with the tests described in the cited publication.
However, under severe conditions of use, for example when it is used as the external layer in motor vehicle glazings where it is subjected to attack, particularly from certain cleaning products containing aggressive organic solvents, it does not given complete satisfaction, since its resistance to these products is not satisfactory.
One solution proposed to improve the resistance of the layer, in particular to solvents, is to provide the layer with a coating layer of a heat-hardenable polyurethane with a modulus of elasticity of approximately 25 to 200 daN/cm2 and a stretching of approximately 100 to 200% with less than 1% plastic deformation.
Another solution, proposed in European Patent Publication No. 0 190 700 consists of "reforming" the external surface of an already formed polyurethane layer by impregnation of the surface of the polyurethane layer with an unsaturated, polymerizable compound, and then polymerizing this compound on the surface of the polyurethane layer. This method, however, is difficult to use because the surface layer formed after polymerization of the unsaturated compound must not exceed 1 micrometer in thickness in order not to destroy the auto-cicatrizing properties of the polyurethane layer. Thus, this method is very delicate. It is therefore necessary to use very precise tempering sequences in order to obtain the desired thickness. In addition, this method is lengthy because it necessitates a cycle for the polymerization of the polyurethane layer and an additional cycle for the polymerization of the acrylic surface layer.
Thus, a need continues to exist for a high optical quality transparent plastic sheet having an energy-absorbing polyurethane layer, which has the necessary properties required for use as an external layer in safety glazings which can be manufactured in a simple manner.